The present invention relates to the preparation of curable pellets of polyethylene or copolymers of polyethylene for use in a continuous vulcanization extrusion coating process.
In order to produce vulcanized extruded articles from curable pellets of polyethylene or its copolymers a curing agent such as an organic peroxide must be uniformly dispersed throughout the polymer prior to the extrusion of the curable polyethylene upon a wire or other article which is being coated therewith. Heretofore, curable pellets of polyethylene and copolymers of polyethylene for use in extrusion coating processes have been prepared by one of the methods: (1) in which polyethylene or copolymers thereof and a curing agent are mixed on a two-roll mill above the melting point of the polyethylene and the resulting blend cooled and repelletized using a pelletizer; (2) in which the polyethylene or its copolymers and a curing agent are first premixed at a temperature below the melting point of polyethylene in a mixer such as a ribbon blender or vortex mixer, then fused and mixed at a temperature above the melting point of polyethylene in a twin screw continuous mixer and subsequently cooled and pelletized; (3) in which polyethylene or copolymers thereof and a curing agent are mixed at a temperature above the melting point of polyethylene using a Banbury mixer, calendered at a temperature above the softening point using a two-roll mill and thereafter the mixture is cooled and pelletized; or (4) in which the polyethylene or copolymers thereof and inert fillers and other additives are fused and mixed at a temperature above the melting point of polyethylene in a twin-screw continuous mixer or Banbury mixer and the mixture is thereafter cooled and pelletized and the curing agent is subsequently added at a later time in a high intensity mixer at a temperature below the softening point of polyethylene by stirring curing agent and pellets at a high speed until the curing agent is evenly dispersed on the pellets and absorbed therein.
The above methods have shortcomings in that the pellets produced are likely to become contaminated during the milling and mixing stages of the process thereby causing the extruded articles produced therefrom to have impaired dielectric properties or the vulcanization reaction can be started during the mixing process if the processing temperature exceeds the decomposition. Moreover, the cost of the equipment used is high and the production rates are low.